The electronics industry is an example of one which makes substantial use of printed wiring boards and substrates as supports and dielectric participants for electronic circuits, such substrates consisting of thin flat pieces produced to exacting specifications as to starting material and physical and electrical properties. The history of the industry shows the use of progressively higher operating frequencies and currently for frequencies up to about 800 megahertz (MHz) copper coated circuit boards of glass fiber reinforced polymers, such as epoxies, cyanide esters, polytetrafluoroethylene (PTFE) and polyimides, have been and are still used. At present one popular laminate material for such applications is FR-4, consisting of epoxy resin deposited on a woven glass fabric, because of its ease of manufacture and low cost. Typically this material has a dielectric constant of 4.3-4.6 and a dissipation factor of 0.016-0.022 and is frequently used in computer related applications below about 500 MHz frequencies. Mobile telephones now operate at frequencies of 1-40 GHz and some computers already at 0.5 GHz, with the prospect of higher frequencies in the future. The lowest possible value of dielectric constant is preferred in computer applications to improve signal speed. At higher operating frequencies above approximately 0.8 GHz, FR-4 and similar materials are materials, despite their low cost, are no longer entirely suitable, primarily because of unacceptable dielectric losses, heating up, lack of sufficient uniformity, unacceptable anisotropy, unacceptable mismatch of thermal expansion between the dielectric material and its metallization, and anisotropic thermal expansion problems as the operating temperatures of the substrates fluctuate. These thermal expansion problems result from the relatively large coefficients of thermal expansion of the polymers used as substrate material, and the unequal expansion coefficients of reinforcing fibers in their length and thickness dimensions. For frequencies above 800 MHz the dielectric material of the substrates become an active capacitative participant in signal propagation and here the current materials of choice are certain ceramics formed by sintering or firing suitable powdered inorganic materials, such as fused silica; alumina; aluminum nitride; boron nitride; barium titanate; barium titanate complexes such as Ba(Mg.sub.1/3 Ti.sub.2/3)O.sub.2, Ba(Zr,Sn)TiO.sub.4, and BaTiO.sub.3 doped with Sc.sub.2 O.sub.3 and rare earth oxides; alkoxide-derived SrZrO.sub.3 and SrTiO.sub.3 ; and pyrochlore structured Ba(Zr,Nb) oxides. Substrates have also been employed consisting of metal powders, and semiconductor powders embedded in a glass or polymer matrix, a particular preferred family of polymers being those based on PTFE.
For example, ceramic substrates that have been used for hybrid electronic circuit applications comprise square plates of 5 cm (2 ins) side, their production usually involving the preparation of a "slip" (slurry) of the finely powdered materials dispersed in a liquid vehicle, dewatering the slip to a stiff leathery mixture, making a "green" preform from the mixture, and then sintering the preform to become the final substrate plate. The substrates are required to have highly uniform values of thickness, grain size, grain structure, density, surface flatness and surface finish, with the purpose of obtaining uniform dielectric, thermal and chemical properties, and also to permit the uniform application to the surfaces of fine lines of conductive or resistive metals or inks.
Such sintered products inherently contain a number of special and very characteristic types of flaws. A first consists of fine holes created by the entrainment of bubbles in the ceramic pre-casting slip of sizes in the range about 1-20 micrometers; these bubbles cannot be removed from the slip by known methods and cause residual porosity in the body. As an example, sintered alumina substrates may have as many as 800 residual bubble holes per sq/cm of surface (5,000 per sq/in). Another flaw is triple-point holes at the junctions of the ceramic granules when the substrate has been formed by roll-compacting of spray-dried powder; they are of similar size to the bubble holes and appear in similar numbers per sq/cm. Yet another consists of "knit-lines", which are webs or networks of seam lines of lower density formed at the contact areas between butting particles during cold pressing. Two other common flaws are caused by inclusions of foreign matter into the material during processing, and the enlarged grains caused by agglomeration of the particles despite their initial fine grinding. The usual inclusions are fine particles due to abrasive wear of the grinding media in the mills. Both inclusions and agglomerates will sinter in a matrix at a different rate from the remainder of the matrix and can result in flaws of much greater magnitude than the original inclusion or agglomerate.
Costly mirror-finishing by diamond machining and lapping of the ceramic surfaces is required to allow the accurate deposition of sputtered metallization layers from which conductor lines are formed by etching. Mirror-finishes are required because the electrical currents at frequencies above 0.8 GHz move predominantly in the skin of a conductor line while in the lower frequencies they occupy the entire crossection of the conductor line. The thickness of the skin through which currents move at GHz frquencies becomes thinner as frequencies rise and are already as thin as 1 to 2 micrometers in copper at around 2 GHz. Any surface roughness of the conductors on the top and bottom sides will therefor contribute to considerable conductive losses. For example, at a frequency of 4 GHz, the conductive loss at of the interface between conductor and substrate is 1.65 times higher at a RMS roughness of 40 compared to a RMS roughness of 5 (See P.42 of Materials and Processes for Microwave Hybrids, Richard Brown, published 1989 by the International Society for Hybrid Microelectronics of Reston, Va.)
There is therefore continuing interest in methods for manufacturing composite materials for the production of electronic substrates and for use as electronic packaging materials, with which sintering and the high processing temperatures required together with their attendant difficulties, high cost of diamond machining and lapping, and the associated considerable costs are avoided.
The low inherent mechanical strength of the currently available matrix forming polymers and their excessive thermal expansion coefficient has made it necessary to embed reinforcing materials, such as woven glass fiber cloth, into the substrate body, to strengthen it and also to contrain its excessive thermal expansion. But such reinforcing materials unfortunately cause unacceptable inhomogenity of the structure. For example, the presence of such reinforcing material makes it difficult to incorporate powdered filler materials uniformly into the body of the substrate. Another difficulty is caused by the generally poor adhesion exhibited by the commercially available matrix polymers toward the usual filler materials, and extensive research and development has been undertaken in the past, and is continuing, in connection with known substrate-forming polymers, such as PTFE, to find coupling agents that will provide adequate adhesion between the polymer and the powder components, and thus satisfactory mechanical strength in the resultant substrates.
Dielectric materials are commonly used as insulating layers between circuits, and layers of circuits in multilayer integrated circuits, the most commonly used of which is silica, which in its various modifications has dielectric constants of the order of 3.0-5.0, more usually 4.0-4.5. Low values of dielectric constant are preferred and organic polymers inherently usually have low dielectric values in the range 1.9-3.5, so that considerable research and work has been done to try to develop polymers suitable for this special purpose, among which are polyimides (frequently fluorinated), PTFE, and fluorinated poly(arylene ethers), some of the materials having dielectric constants as low as that of air, i.e. 1.00. At the present time fluorination is the most common modification of the polymers employed in view of the improvements obtained comprising lowered dielectric constants, enhanced optical transparency, and reduced hydrophilicity and solubility in organic solvents, but the fluorination usually results in the polymers exhibiting a degree of polarization which can cause problems in obtaining the desired dielectric properties.
U.S. Pat. No. 5,658,994, issued Aug. 19, 1997, and U.S. Pat. No. 5,874,516, issued Feb. 23, 1999, both to Air Products and Chemicals, Inc. of Allentown, Pa., the disclosures of which are incorporated herein by this reference, disclose and claim a unique utility as a dielectric coating material for micro-electronic devices of a class of poly(arylene ethers) as a replacement for silica-based dielectric materials, wherein the poly(arylene ether) does not have nonaromatic carbons (other than perphenylated carbon), fluorinated substituents or significantly polarizable functional groups. These materials, which are relatively easily synthesized, are found surprisingly to have an excellent combination of desirable properties, namely thermal stability, low dielectric constant values, low moisture absorption and low moisture outgassing.
U.S. Pat. No. 5,658,994 discloses and claims in its broadest aspect an article of manufacture comprising a combination of a dielectric material and a microelectronic device, wherein the dielectric material is provided on the microelectronic device and contains a poly(arylene ether) polymer consisting essentially of non-functional repeating units of the structure: EQU --{--O--Ar.sub.1 --O--Ar.sub.2 --}.sub.m --{--O--Ar.sub.3 --O--Ar.sub.4 --}.sub.n --
wherein m=0 to 1.0; and n=1.0-m; and Ar.sub.1, Ar.sub.2, Ar.sub.3 and Ar.sub.4 are individually divalent arylene radicals selected from the group consisting of: phenylene; biphenyl diradical; para-terphenyl diradical; meta-terphenyl diradical; ortho-terphenyl diradical; naphthalene diradical; anthracene diradical; phenanthrene diradical; diradicals of 9,9-diphenylfluorene of specific type; and 4,4'-diradical of dibenzofuran and mixtures thereof, but Ar.sub.1, Ar.sub.2, Ar.sub.3, and Ar.sub.4, other than the diradical 9,9-diphenylfluorene, are not isomeric equivalents. PA1 wherein m=0.2 to 1.0; and n=1.0-m; and Ar.sub.1, Ar.sub.2, and Ar.sub.3 are individually divalent radicals selected from the group defined in the preceding paragraph; or essentially of non-functional repeating units of the structure: EQU {--O--Ar.sub.x --O--Ar.sub.1 --}.sub.m --{--O--Ar.sub.x --O--Ar.sub.3 --}.sub.n -- PA1 wherein m=0 to 1.0; and n=1.0--m; Ar.sub.x is a special radical 9,9-bis(4-oxyphenyl)fluorene and Ar.sub.1, and Ar.sub.3 are individually divalent radicals also selected from the group defined in the immediately preceding paragraph. PA1 mixing together from 60 to 97 volume percent of particles of the filler material of minimum pore volume when compacted and the balance of polymer bonding material consisting of nonfunctionalized poly(arylene ether) to form a composite mixture; and PA1 subjecting the composite mixture to a temperature sufficient to melt the polymer material and to a pressure sufficient to uniformly disperse the melted polymer material into the interstices between the particles of filler material. PA1 from 60 to 97 volume percent of particles of the compacted filler material and the balance of polymer material consisting of nonfunctionalized poly(arylene ether) together forming a uniform composite mixture; PA1 wherein the composite mixture has been subjected to a temperature sufficient to melt the polymer material and to a pressure sufficient to uniformly disperse the melted polymer material into the interstices between the particles of filler material.
U.S. Pat. No. 5,874,516 claims poly(arylene ether) consisting essentially of non-functional repeating units of the structure: EQU --{--O--Ar.sub.x --O--Ar.sub.1 --}.sub.m --{--O--Ar.sub.2 --O--Ar.sub.3 --}.sub.n --
Variations in Ar.sub.1, Ar.sub.2, Ar.sub.3 and Ar.sub.4 are stated to allow access to a variety of properties such as reduction or elimination of crystallinity, modulus, tensile strength, high glass transition temperature, etc. The polymers are said to be essentially chemically inert, have low polarity, have no additional functional or reactive groups, and to be thermally stable at temperatures of 400.degree.-450.degree. C. in inert atmospheres. In addition to the basic polymer structures as outlined above the polymers may also be cross-linked, either by cross-linking itself, through exposure to temperatures in the range of 350.degree.-450.degree. C., or by providing a cross-linking agent, as well as end capping the polymer with known end capping agents, such as phenylethynyl, benzocyclobutene, ethynyl and nitrile. The ability to crosslink at elevated temperatures, with the consequent marked increase in molecular weight and density makes the materials particularly useful in microelectronic applications because they can readily be applied from solution and then converted to a solvent resistant coating by heating.
The specified polymers are non-functional in that they are chemically inert and they do not bear any functional groups that are detrimental to their application in the fabrication of microelectronic devices. They do not have carbonyl moieties such as amide, imide and ketone, which promote adsorption of water. They do not bear halogens such as fluorine, chlorine, bromine and iodine which can react with metal sources in metal deposition processes. They are composed essentially of aromatic carbons, except for the bridging carbon in the 9,9-fluorenylidene group, which has much of the character of aromatic carbons due to its proximity to aromatic structures; for the purposes of the invention the carbon is deemed to be a perphenylated carbon.
The polymers are proposed for use as coatings, layers, encapsulants, barrier regions or barrier layers or substrates in microelectronic devices. These devices may include, but are not limited to multichip modules, integrated circuits, conductive layers in integrated circuits, conductors in circuit patterns of an integrated circuit, circuit boards as well as similar or analogous electronic structures requiring insulating or dielectric regions or layers. They are also proposed for use as a substrate (dielectric material) in circuit boards or printed wiring boards. Such a circuit board has mounted on its surface patterns for various electrical conductor circuits, and may include various reinforcements, such as woven nonconducting fibers, such as glass cloth. Such circuit boards may be single sided as well as double sided or multilayer.
It is proposed that additives can be used to impart particular target properties, as is conventionally known in the polymer art, including stabilizers, flame retardants, pigments, plasticizers, surfactants, and the like. It is also proposed that adhesion promoters can be used to adhere the polymers to the appropriate substrates. Such promoters are typified by hexamethyldisilazane, which can be used to interact with available hydroxyl functionality that may be present on a surface, such as a silica surface.